Simultaneous Warm Gas Desulfurization and CO-Shift for Improved Syngas Cleanup

ABSTRACT

The present invention involves a process and materials for simultaneous desulfurization and water gas shift of a gaseous stream comprising contacting the gas stream with a nickel aluminate catalyst. The nickel aluminate catalyst is preferably selected from the group consisting of Ni 2x Al 2 O 2x+3 , Ni (2−y) Ni 0   y Al 2 O (5−y) , Ni (4−y) Ni 0   y Al 2 O (7−y) , Ni (6−y) Ni 0   y Al 2 O (9−y) , and intermediates thereof, wherein x≧0.5 and 0.01≦y≦2. Preferably, x is between 1 and 3. More preferably, the nickel containing compound further comprises Ni 2x Al 2 O 2x+3−z S z  wherein 0≧z≧2x.

BACKGROUND OF THE INVENTION

This invention relates to the materials used for and process for boththe removal of sulfur compounds from a gaseous stream and for the watergas shift reaction. More specifically, the invention relates to the useof a nickel containing aluminate catalyst to provide for simultaneousdesulfurization and water gas shift at temperatures of about 450° C.

The gaseous stream may originate from any partial oxidation orgasification process of a carbon containing feedstock. The gaseousstream may be a fuel gas originating from an IGCC (IntegratedGasification Combined Cycle) coal gasification plant, it may be a fluegas from a fluid catalytic cracking unit (FCC), it may be a synthesisgas (syngas) from steam reforming of natural gas, certain gasificationreactions or from gasification of coal. Synthesis gas is the namegenerally given to a gaseous mixture principally comprising carbonmonoxide and hydrogen, but also possibly containing carbon dioxide andminor amounts of methane and nitrogen.

Synthesis gas is used, or is potentially useful, as feedstock in avariety of large-scale chemical processes, for example: the productionof methanol, the production of gasoline boiling range hydrocarbons bythe Fischer-Tropsch process and the production of ammonia.

Processes for the production of synthesis gas are well known andgenerally comprise steam reforming, auto-thermal reforming,non-catalytic partial oxidation of light hydrocarbons or non-catalyticpartial oxidation of any hydrocarbons. Of these methods, steam reformingis generally used to produce synthesis gas for conversion into ammoniaor methanol. In such a process, molecules of hydrocarbons are brokendown to produce a hydrogen-rich gas stream.

Regardless of the carbon source and gasification process, the generatedfuel gas has to be substantially cleaned before being either burned in agas turbine or used for chemical synthesis, e.g., methanol, ammonia,urea production, Fischer-Tropsch synthesis. The clean-up of hot fuelgases avoids the sensible heat loss due to the cooling and subsequentreheating associated with the wet scrubbing techniques using eitherchemical or physical solvents. Ideally, the clean-up of the fuel gas isdone at the highest temperature that the fuel gas distribution systemcan be designed at. This could improve greatly the overall processefficiency, however, there are significant hurdles that need to beovercome before such a hot-fuel gas clean-up system is made commerciallyavailable. Only the hot particulate removal systems, i.e., candlefilters or sintered metal filters, have been successfully demonstratedcommercially for long term applications in a temperature range of 200°to 250° C. at the Nuon's Shell coal gasification plant in TheNetherlands, and 370° to 430° C. in the E-Gas coal/coke gasificationsystem at the Wabash River plant. All large scale warm desulfurizationdemonstration units have failed mostly due to inappropriatesulfur-scavenger materials.

Both large scale warm gas desulfurization units (Pinon Pine Air-BlownIGCC and Tampa Electric Polk Power station) used Zn-based S-scavengermaterials. The Pifion Pine Air-Blown and Hot Gas Cleanup IGCC using aKRW air-blown pressurized fluidized-bed coal gasification system withSouthern Utah bituminous coal containing 0.5-0.9% sulfur (design coal)and Eastern bituminous coal containing 2-3% sulfur (planned test). Thepurpose was to demonstrate air-blown, pressurized, fluidized-bed IGCCtechnology incorporating hot gas cleanup (HGCU); to evaluate a low-Btugas combustion turbine; and to assess long-term reliability,availability, maintainability, and environmental performance at a scalesufficient to determine commercial potential. Steady state operation wasnot reached in the course of the 42 months demo operation and theZn-based S-scavenger material failed since it did not hold up physicallyin the entrained bed reactor. Zn was lost during the 538° C. reactionvia volatilization. The second large scale hot gas desulfurization demounit at Tampa Electric Polk Power station intended to clean 10% of thefuel gas by a hot-gas cleanup system developed by GE EnvironmentalServices, Inc. The hot gas desulfurization unit was an intermittentlymoving bed of Zn oxide based sorbent that operated at 482° C. Thedemonstration again failed due to very high attrition loss (which madeoperation with that particular sorbent far from cost effective) and dueto significant reactivity loss because of Zn sulfate formation and Znvolatilization. (References: The Pinon Pine IGCC Project, U.S. DOE andPinon Pine Power Project Reports, December 1996; January 2001(DE-FC-21-92MC29309). The Tampa Electric IGCC Project, U.S. DOE andTampa Electric Reports, October 1996; July 2000; August 2002(DE-FC-21-91MC27363))

Some patents that disclose the use of Zn-containing S-sorbents includeseveral assigned to Phillips Petroleum: U.S. Pat. No. 5,045,522; U.S.Pat. No. 5,130,288; U.S. Pat. No. 5,281,445; U.S. Pat. No. 5,306,685;and U.S. Pat. No. 6,479,429. There are also several patents assigned toRTI (Research Triangle Institute): U.S. Pat. No. 5,254,516; US2004/0170549 A1; and U.S. Pat. No. 7,067,093. There is no priordisclosure of the simultaneous removal of S-compounds from a gaseousstream and the water gas shift reaction.

With the current state of development of hot gas cleanup systems, allthe other contaminants besides the S-compounds and solid particulatescan not be removed at equally high temperatures. Even more, due to theimminent CO₂ regulations, all integrated gasification combined cycle(IGCC) gasifiers will have to be equipped with at least one CO-shiftreactor, requiring thus cooling the fuel gas to temperatures adequatefor the water gas shift catalytic reaction. In view of these CO₂regulations, the trend in the gasification industry is towards use ofdirect water quench gasifiers. The quench mode design significantlyreduces the capital cost of syngas cooling, while heat integrationmaintains good overall thermal efficiency. The quench mode isadvantageous for the water gas shift reaction as the raw syngas becomessaturated with steam generated by evaporation of a portion of the quenchwater. An entrained-flow slurry-fed gasification with direct waterquenching is the preferred and commonly used option of GE Energy, andrecently, Shell, Lurgi and Siemens also offer the water quenchingcooling method. In addition to efficiently cooling the raw syngas andrecovering part of the sensible heat, significant decontamination takesplace in the quenching step. Solid particulates, alkali metals,non-volatile metals, chlorides, the bulk of metal carbonyls and part ofammonia are all removed in the water quenching step. The contaminantsleft in the raw syngas after the water quenching step include about50-100 ppmv ammonia, 1 to 4 ppmv Ni and Fe carbonyls, about 50-100 ppmvHCN, Hg, As, and sulfur-containing gases, i.e., H₂S and COS. All thesecontaminants must be removed before the syngas is either burned in a gasturbine or used for chemical synthesis.

This invention discloses a class of materials able to simultaneouslycompletely desulfurize (remove H₂S and COS) a fuel gas originating froma gasification process and shift the CO to C0₂ in a temperature range of250° to 550° C. The CO₂ stream can be further shifted by adding anadditional sweet CO-shift unit downstream of this integrateddesulfurization and CO-shift unit. Thus, the hydrogen production ismaximized and the clean, concentrated CO₂ stream can be captured usingeither a physical solvent process, e.g., UOP's Selexol process, oralternatively using high temperature CO₂ absorbents. This integrateddesulfurization and CO-shift concept represents the next generation ofsynthesis gas treating. Currently, regenerable solvent-type acid gasremoval processes are used in both IGCCs and chemical synthesisapplications, e.g., UOP's Selexol process (U.S. Pat. No. 2,649,166 andU.S. Pat. No. 3,363,133) or Linde Engineering's Rectisol process (U.S.Pat. No. 2,863.5277). Unfortunately, these processes require cooling thefuel gas to low temperatures and then subsequently reheating it totemperatures adequate to its downstream use. This issue associated withthe solvent scrubbing based clean-up processes can be addressed by usingthe concept disclosed in this invention. This invention relates to theuse of a nickel containing aluminate catalyst to provide forsimultaneous desulfurization and water gas shift at temperatures ofabout 450° C. The CO₂ stream can be further concentrated (completeCO-shift) by adding an additional sweet CO-shift unit downstream of thisintegrated unit. Thus, the hydrogen production is maximized and theclean, concentrated CO₂ stream can be captured using either a physicalsolvent process, or alternatively using high temperature CO₂ absorbents.There are several main advantages associated with this concept: bycontinuously removing the H2S from the gaseous stream, the COShydrolysis equilibrium is shifted completely to the right, the CO₂stream is concentrated via the water gas shift reaction, and alsopossibly the equipment costs could be greatly reduced.

SUMMARY OF THE INVENTION

The present invention involves a process for simultaneousdesulfurization and gas shift of a gaseous stream comprising contactingthe gaseous stream with a nickel aluminate catalyst. The nickelaluminate catalyst is preferably selected from the group consisting ofNi_(2x)Al₂O_(2x+3), Ni_((2−y))Ni⁰ _(y)Al₂O_((5−y)), Ni_((4−y))Ni⁰_(y)Al₂O_((7−y)), Ni_((6−y))Ni⁰ _(y)Al₂O_((9−y)), and intermediatesthereof, wherein x≧0.5 and 0.01≦y≦2. Preferably, x is between 1 and 3.More preferably, the nickel containing compound further comprisesNi_(2x)Al₂O_(2x+3−z)S_(z) wherein 0≦z≦2x. The nickel containing compoundreacts with more than 10% of sulfur compounds within the gaseous stream.Preferably, the nickel containing compound reacts with more than 50% ofthe sulfur compounds within the gaseous stream. At least 10% of thecarbon monoxide within the gaseous stream is converted to carbondioxide. The nickel containing compound contacts the gaseous stream at atemperature from about 250° to 550° C. Preferably the temperature isfrom 400° to 500° C. and the pressure is from about 10 to 80 bar. TheGHSV (at STP) is preferably higher than 500 m³/m³/hr. Steam and CO arepresent at a molar ratio from about 0.5:1 to 4:1 and preferably at amolar ratio from about 1.5: 1 to 3.5:1.

The gaseous stream is produced by gasification of hydrocarbons,including fuel gas and syngas.

The invention also involves the use of this catalyst in the treatment offuel gas comprising carbon monoxide, hydrogen, carbon dioxide, sulfurcompounds and various impurities.

DETAILED DESCRIPTION OF THE INVENTION

We disclose here a class of materials able to desulfurize (H₂S and COScomplete removal) fuel gases and simultaneously shift CO to CO₂. Theclass of materials consists of nickel aluminates prepared fromhydrotalcite as the starting material. The nickel aluminate catalyst ispreferably selected from the group consisting of Ni_(2x)Al₂O_(2x+3),Ni_((2−y))Ni⁰ _(y)Al₂O_((5−y)), Ni_((4−y))Ni⁰ _(y)Al₂O_((7−y)),Ni_((6−y))Ni⁰ _(y)Al₂O_((9−y)), and intermediates thereof, wherein x≧0.5and 0.01≦y≦2. Preferably, x is between 1 and 3. More preferably, thenickel containing compound further comprises Ni_(2x)Al₂O_(2x+3−z)S_(z)wherein 0≦z≦2x. An Ni aluminate material (Ni 4.09: Al₂O 7.09: 2.4 H₂O)was shown to have excellent S absorption capacity, i.e., 10 wt-% Sbefore pre-breakthrough, and simultaneously achieve 40-50% CO conversionto CO₂. The materials are regenerable.

In addition, herein we disclose a class of materials used for and aprocess used for the complete hydrolysis and hydrogenation of COS to H₂Sin a sulfur containing gaseous stream, said materials and process usedfor the complete removal of said H₂S from said gaseous stream and saidmaterials and process for shifting CO to CO₂. The nickel aluminatecatalyst is preferably selected from the group consisting ofNi_(2x)Al₂O_(2x+3), Ni_((2−y))Ni⁰ _(y)Al₂O_((5−y)), Ni_((4−y))Ni⁰_(y)Al₂O_((7−y)), Ni_((6−y))Ni⁰ _(y)Al₂O_((9−y)), and intermediatesthereof, wherein x≧0.5 and 0.01≦y≦2. Preferably, x is between 1 and 3.More preferably, the nickel containing compound further comprisesNi_(2x)Al₂O_(2x+3−z)S_(z) wherein 0≦z≦2x.

EXAMPLE 1

An Ni aluminate material with the final formula Ni_(4.09) Al₂ O_(7.09):2.4 H₂O was prepared via a Layered Double Hydroxide (LDH) Metal OxideSolid Solution (MOSS) route. In this procedure, a clear solution wasprepared by combining 328.0 g of 50% aqueous NaOH with 1170.0 g DIwater, followed by the addition of 136.1 g of NaCO₃:H₂O. A secondsolution was prepared by dissolving 345.3 g Ni(NO₃)₂:6H₂O and 217.7 gAl(NO₃)₃:9H₂O into 840.0 g DI water. The aqueous metal nitrate solutionwas then added to the first solution dropwise while stirring over a 2hour period. The reaction mixture was heated to 80° C. and held attemperature for 16 to 18 hours while stirring. The solids were thenisolated by vacuum filtration and washed vigorously with DI water (26liters) and dried in ambient air. At this point, XRD confirmed thesynthesis of Ni—Al—O Layered Double Hydroxide material which was thencalcined (in flowing air) for 6 hours at 450° C. followed by 4 hours at550° C. to generate the metal oxide solid solution. The resultingmaterial contained 58.5 wt-% Ni, had a surface area of 189 m²/g, porevolume=0.337 cm³/g and pore diameter=71 Å.

EXAMPLE 2

The sulfidation/CO-shift experiments were done with the Ni_(4.09) Al₂O_(7.09): 2.4 H₂O material prepared in Example 1 at atmosphericpressure, 450° C. with a gas simulating an oxygen blown gasifiercontaining 1.1% H₂S+0.0763% COS+45% H₂+46% CO+7.2% CO₂+0.7% CH₄. Steamhad been co-fed in a steam:CO molar ratio 3.5:1. The wet basis GHSV was2100 h⁻¹. The oxidative regeneration was done with 2% O₂ in N₂ at 500°C. and GHSV=2100 h⁻¹. In the very first cycle, the S absorption capacitybefore S-breakthrough was 10 wt-% S and the CO conversion was about 95%to both CO₂ and CH₄. The Ni aluminate generated about 10% CH₄ whichrepresents about 60% of the equilibrium methane formation. The samplehad been heated to the reaction temperature under a H₂ atmosphere, suchthat some of the Ni present in the Ni aluminate material had beenreduced to the metallic state thus creating the active sites for themethanation reaction. After a 500° C. oxidation cycle, when no Ni⁰ whichpromotes the methanation reaction is present in the nickel aluminatematerial, no CH₄ formation was observed. The CO conversion to CO₂ wasabout 50%. However, the S absorption capacity before S-breakthrough wasstill 10 wt-% S. Methane formation is undesirable since it contributesto the non-captured carbon inventory; however, as it will be shownbelow, the yield to methane in all subsequent cycles is zero.

In the first cycle, the amount of methane formed decreased continuouslyas the Ni metal was progressively being sulfided. Without wanting to bebound by any theory, we believe that the S-compounds present in the feedinhibit the methanation reaction since they adsorb on the same sites,i.e., Ni⁰, as H₂ would otherwise adsorb reducing thus the hydrogenationactivity. Sulfur poisons the ability of Ni-catalysts to hydrogenate thecarbon atom much more severely than the ability to form carbon-carbonbonds. However, since the feed contains a large amount of steam, the C—Cbond formation is also inhibited, such that the only reaction that takesplace involving the C-containing compounds is the water gas shiftreaction. The materials were regenerated via oxidative regeneration at500° C. under 2% O₂ in N₂. Only sulfur dioxide was detected during theoxidation step.

In the second cycle, after a 500° C. oxidative regeneration, thematerial recovered completely the S-absorption capacity but recoveredonly about 42% of the CO conversion activity. No CH₄ was formed in thesecond cycle. The methane formation is completely inhibited due to theabsence of Ni^(o) and also probably due to a S poisoning effect by the0.5 to 1.5 wt-% residual S left behind after the regeneration step. Thepresence of the residual S was confirmed via the S-XRF analysis of theKOH scrubbing solution and by XAFS analysis. As indicated before, sulfuradsorbs strongly on the sites that H₂ would otherwise adsorb reducingthus the hydrogenation activity of the catalyst. The fact that theresidual S (from a previous sulfidation cycle) might poison the sitesresponsible for methanation is indicated by the complete inhibition ofCH₄ formation, but similar CO-shift to CO₂ conversion during the secondsulfidation cycle after an oxidative regeneration followed by a H2pretreatment step. One would expect that a H₂ treatment would reducesome nickel to Ni⁰ creating thus the active sites for methanation.However, the methane yield is zero while the CO-shift conversion to CO₂is similar to the conversion achieved in the absence of such apre-reduction step. This indicates that the sites responsible formethanation (Ni⁰) are poisoned by sulfur while the Ni oxide (generatedafter the oxidation cycle) sites are available for both CO-shift and Suptake. In the second cycle, the S uptake before pre-breakthrough was 10wt-% S. The CO to CO₂ shift activity of the Ni material remainsconstant, at about 40-50% CO conversion to CO₂ even after the materialis sulfided. Nickel sulfide is known to be a sour CO-shift catalyst.

Another reason for the decreased CO-shift activity could be the physicalchange occurring in the MOSS material after sulfidation-oxidationtreatments. After two oxidation cycles however, the material lost about50% of its surface area (Table 1), suggesting that some MOSS structuremight have collapsed. The narrow NiO peaks identified via XRD suggestthat possibly some metal might have migrated creating larger sinteredmetal oxide crystallites. The fresh MOSS material calcined at 450° C.has the same surface area as the fresh material calcined at 550° C.,suggesting that the heat treatment alone does not destroy the MOSSstructure, but the sulfidation-oxidation treatments promote some surfacearea loss and surface roughening.

TABLE 1 Surface area, pore volume and pore diameter for the Ni Aluminatematerial exposed to different temperatures and atmospheres PV MaterialSA (m²/g) (cm³/g) PD (Å) Fresh Ni Aluminate Calcined at 450° C. 1890.337 71 Fresh Ni Aluminate Calcined at 550° C. 184 0.355 77 Fresh NiAluminate - pretreated in H2 at 153 0.316 83 450° C. 2 CyclesSulfided/Oxidized 95 0.29 124

Table 2 summarizes the Ni aluminate performance for desulfurization andCO-shift. The yields to CO₂ and CH₄ were calculated according to theequations:

$Y_{{CO}_{2}} = {\frac{{\% \begin{matrix}{out} \\{CO}_{2}\end{matrix}} - {\% \begin{matrix}{in} \\{CO}_{2}\end{matrix}}}{\% \begin{matrix}{in} \\{CO}\end{matrix}}\mspace{14mu} {and}}$$Y_{{CH}_{4}} = \frac{{\% \begin{matrix}{out} \\{CH}_{4}\end{matrix}} - {\% \begin{matrix}{in} \\{CH}_{4}\end{matrix}}}{{\% \begin{matrix}{in} \\{CO}\end{matrix}} + {\% \begin{matrix}{in} \\{CO}_{2}\end{matrix}}}$

where: X_(CO)=carbon monoxide conversion (%); Y_(CO2)=yield to carbondioxide (%); Y_(CH4)=yield to methane (%); %_((CO2, CO, CH4)) ^(in)refers to the volume % of CO2, CO or CH4 in the feed gas and%_((CO2, CO, CH4)) ^(out) refers to the volume % of CO₂, CO or CH₄ inthe exhaust gas.

TABLE 2 Performance of the Ni aluminate (Ni/Al = 2) for warm gasdesulfurization and CO conversion Minutes Wt-% S Sulfidation on stream(up to Y_(CO2) Y_(CH4) Material Cycle wt. no S breakthrough) X_(CO) (%)(%) (%) Ni/Al = 2 1 280 10 96 → 66 84 → 63 17 → 12 2 280 10 45 30 0 1280 10 45 30 0 After an oxidation cycle 2 280 10 45 30 0

EXAMPLE 3 COMPARATIVE EXAMPLE

A sample of Ni loaded alumina was prepared by standard impregnationtechnique. A solution containing 14.83 g of Ni(NO3)3:6H2O dissolved into30 DI H2O was prepared. To this solution, 10.7 g of R-50/R51 gammaalumina extrudates were added while stirring gently. Stirring at roomtemperature was continued for 20 hours after which the solution wasevaporated with occasional stirring in an 80° C. forced air oven toyield dry solids. The resultant dried solids were then calcined inflowing air with 3° C./min heating rate to 550° C. and held attemperature for 6 hours. The resultant product was measured by ICP tocontain 38.3% Al, 21.1% Ni with an LOI at 900° C.=12.84% giving a finalproduct formula of Ni_(0.51) Al₂ O_(3.51): 1.0 H₂O. N₂ BET surface areawas measured at 163 m²/g with a pore volume=0.361 cc/g. This finishedmaterial was identified by XRD analysis to contain NiO and gammaalumina. The NiO on gamma-alumina was tested under the conditionsdescribed in Example 2. While the CO conversion to CO₂ was similar tothe Ni aluminate material, however, the S absorption capacity beforeS-breakthrough was only 3.9 wt-% S.

TABLE 3 Performance of the Ni impregnated on alumina catalyst for warmgas desulfurization and CO conversion Minutes Wt-% S Sulfidation onstream (up to Y_(CH4) Material Cycle wt. no S breakthrough) X_(CO) (%)Y_(CO2) (%) (%) 30% Ni 1 75 3.9 50 25 1.5 impregnated on Al2O3

1. A process for removal of sulfur from a gaseous stream, comprisingcontacting said stream with a nickel containing compound.
 2. The processof claim 1 wherein when the nickel containing compound removes saidsulfur from said gaseous stream, said nickel containing compoundsimultaneously catalyses a water gas shift reaction.
 3. The process ofclaim 2 wherein said gaseous stream comprises carbon monoxide, carbondioxide, hydrogen, and sulfur compounds.
 4. The process of claim 2wherein the gaseous stream comprises a fuel gas.
 5. The process of claim2 wherein the gaseous stream comprises a synthesis gas comprisinghydrogen, carbon monoxide, sulfur containing compounds and impurities.6. The process of claim 2 wherein said nickel containing compound isselected from the group consisting of Ni_(2x)Al₂O_(2x+3), Ni_((2−y))Ni⁰_(y)Al₂O_((5−y)), Ni_((4−y))Ni⁰ _(y)Al₂O(_(7−y)), Ni(_(6−y))Ni⁰_(y)Al₂O_((9−y)), and intermediates thereof, wherein x≧0.5 and 0.01≦y≦2.7. The process of claim 6 wherein 1≦x≦3.
 8. The process of claim 7wherein the nickel containing compound further comprisesNi_(2x)Al₂O_(2x+3−z)S_(z) wherein 0≦z≦2x.
 9. The process of claim 2wherein said nickel contained compound reacts with more than 10% ofsulfur compounds within said gaseous stream.
 10. The process of claim 2wherein said nickel containing compound reacts with more than 50% ofsulfur compounds within said gaseous stream.
 11. The process of claim 2wherein about 10% of said carbon monoxide within said gaseous stream isconverted to carbon dioxide.
 12. The process of claim 2 wherein saidnickel containing compound contacts said gaseous stream at a temperaturefrom about 250° to 550° C.
 13. The process of claim 2 wherein saidnickel containing compound contacts said gaseous stream at a temperaturefrom about 400° to 500° C.
 14. The process of claim 2 wherein saidnickel containing compound contacts said gaseous stream at a pressurefrom about 10 bar to 80 bar.
 15. The process of claim 2 wherein the GHSV(at STP) is higher than 500 m³/m³/hr.
 16. The process of claim 2 whereinsteam and CO are present at a molar ratio from about 0.5:1 to 4:1. 17.The process of claim 2 wherein steam and CO are present at a molar ratiofrom about 1.5:1 to 3.5:1.